1. Field of the Invention
The present invention relates generally to a communication system supporting an Orthogonal Frequency Division Multiple Access (OFDMA) scheme (hereinafter referred to as an “OFDMA communication system”), and in particular, to an apparatus and method for adaptively assigning subchannels.
2. Description of the Related Art
In the late 1990's, South Korea partially deployed a third generation (3G) mobile communication system supporting IMT-2000 (International Mobile Telecommunication-2000), intending to advance wireless multimedia service, worldwide roaming, and high-speed data service. The 3G mobile communication system was developed specifically to transmit data at a higher rate along with the rapid increase of serviced data amount.
The 3G mobile communication system is currently evolving into a fourth generation (4G) mobile communication system. The 4G mobile communication system is being standardized for the purpose of efficient interworking and integrated service between a wired communication network and a wireless communication network, beyond a simple wireless communication service, which the previous-generation mobile communication systems provided. Accordingly, technology for transmitting a large volume of data at a same level available in the wired communication network must be developed for the new wireless communication network.
In this context, many studies are being conducted on using an Orthogonal Frequency Division Multiplexing (OFDM) scheme as a scheme for high-speed data transmission over wired/wireless channels in the 4G mobile communication system. The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal subcarriers (or subcarrier channels) before being transmitted.
The first MCM systems appeared in the late 1950's for military high frequency (HF) radio communication, and the OFDM scheme for overlapping orthogonal subcarriers was initially developed in the 1970's. In view of orthogonal modulation between multiple carriers, the OFDM scheme has limitations in actual implementation. In 1971, Weinstein, et al. proposed that OFDM modulation/demodulation can be efficiently performed using Discrete Fourier Transform (DFT), which was a driving force behind the development of the OFDM scheme. Also, the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates adverse effects of multipath propagation and delay spread on systems. As a result, the OFDM scheme has been widely used for digital data communication technologies such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM).
Although hardware complexity was an obstacle to widespread implementation of the OFDM scheme, recent advances in digital signal processing technology including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) enable the OFDM scheme to be implemented.
The OFDM scheme, similar to an existing Frequency Division Multiplexing (FDM) scheme, boasts of optimum transmission efficiency in high-speed data transmission because it transmits data on subcarriers, while maintaining orthogonality among them. The optimum transmission efficiency is further attributed to good frequency use efficiency and robustness against multipath fading in the OFDM scheme. More specifically, overlapping frequency spectrums lead to efficient frequency use and robustness against frequency selective fading and multipath fading. The OFDM scheme reduces effects of intersystem interference (ISI) by use of guard intervals and enables design of a simple equalizer hardware structure. Furthermore, because the OFDM scheme is robust against impulse noise, it is increasingly popular in communication systems.
The OFDMA scheme is a Multiple Access scheme based on the OFDM scheme. In the OFDMA scheme, subcarriers in one OFDM symbol are distributed to a plurality of users, or subscriber stations. A communication system using the OFDMA scheme includes an IEEE 802.16a communication system and an IEEE 802.16e communication system. The IEEE 802.16a communication system is a fixed-Broadband Wireless Access (BWA) communication system using the OFDMA scheme. The IEEE 802.16e communication system is a system that considers the mobility of subscriber stations in the IEEE 802.16a communication system. Currently, the IEEE 802.16a communication system and the IEEE 802.16e communication system both use 2048-point IFFT and 1702 subcarriers. The IEEE 802.16a communication system and the IEEE 802.16e communication system use 166 subcarriers among the 1702 subcarriers as pilot subcarriers, and use 1536 subcarriers, not including the 166 subcarriers, as data subcarriers.
The 1536 data subcarriers are divided into 32 subchannels, each having 48 data subcarriers. The subchannels are assigned to a plurality of users according to system conditions. The term “subchannel” refers to a channel comprised of a plurality of subcarriers. Herein, each subchannel is comprised of 48 subcarriers. The OFDMA communication system distributes all subcarriers, particularly, data subcarriers used therein over the entire frequency band, thereby acquiring frequency diversity gain.
The IEEE 802.16a communication system and the IEEE 802.16e communication system divide a broadband of, for example, 10 [MHz] into subchannels only in a frequency domain. As indicated above, the IEEE 802.16a communication system and the IEEE 802.16e communication system use 2048-point IFFT and thus use 1702 subcarriers per OFDM symbol. Therefore, when subchannels are assigned using Reed Solomon (RS) sequences, which secures an excellent inter-subchannel collision characteristic in a multi-cell environment, it is possible to identify about 40 cells (e.g., 41*40=1640).
However, in order to facilitate network design along with the development of communication systems, it is necessary to increase the number of identifiable cells up to 100. The OFDMA scheme has limitations in generating subchannels only in a frequency domain in terms of the number of identifiable cells. Further, a Flash-OFDM scheme using a narrowband of 1.25 [MHz] uses 128-point IFFT, and defines 112 hopping sequences that hop different subcarriers for one period comprised of 113 OFDM symbols, as a basic resource assignment unit. A communication system supporting the Flash-OFDM scheme (hereinafter referred to as a “Flash-OFDM communication system”) defines different hopping frequencies for 113 cells in designing networks, thereby making it possible to identify 113 different cells. However, the Flash-OFDM scheme, being a narrowband-only scheme, cannot contribute to the required capacity increase.